If you have backed up via certain tools, the backup files can be extracted easily and restored to a new device. But if you simply want to transfer files stored in PDF Reader to another device, you could try the following steps:

• Zip Files 

1. Switch to the Document tab for the file list
2. Select files and move them to one single folder
3. Tap “Edit” on the top right corner
4. Select the folder and choose “Zip” to compress the folder into a zip file

• Transfer to Desktop 

You could transfer the zip file from your Android device to PC/Mac by connecting to your computer and then restore it to a new device. Or you could choose to sync the zip file to another device through Kdan Cloud or other cloud storage services.